The present invention relates to a measuring apparatus and method of measuring a characteristic of a solar cell and, more particularly, to a measuring apparatus and method of measuring a photoelectric conversion characteristic of a solar cell having a broader photo-sensing area than an illuminated area by a light source.
Photovoltaic power generation has been collecting interests of many people as a clean power generation method which will meet an increasing demand for electric power and does not cause the destruction of the environment, since it does not cause environmental problems, such as radioactive contamination and anathermal of the earth, further, sunlight falls everywhere on the earth with some distribution inequality, and relatively large power generation efficiency is realized without a complicated and large facility. Accordingly, various studies and development are made on photovoltaic power generation to fit for practical use.
Upon studying and developing a solar cell, not only manufacturing technique of a solar cell, but also technique for evaluating the output characteristic of a manufactured solar cell are very important items. As a method of evaluating an output characteristic of a solar cell, a method of studying a voltage vs. current characteristic (voltage/current characteristic) of a solar cell is generally used. FIG. 2 shows a configuration of an apparatus for evaluating the voltage/current characteristic. In FIG. 2, reference numeral 201 denotes a solar cell which is the object of evaluation; 202, a direct current (DC) power source; 203, wires; 204, an ammeter; 205, a voltmeter; 206, wires used for measuring voltage; 207, a computer; 208, a light source; and 209, a shutter.
The DC power source 202, controlled by the computer 207, is connected to the solar cell 201 via the wire 203 and the ammeter 204. Generally, a bipolar DC power source is used as the DC power source 202; however, an electronic load may be used instead. Further, the voltmeter 205 is connected across the solar cell 201. The values of current and voltage measured by the ammeter 204 and the voltmeter 205, respectively, are inputted to the computer 207. The light source 208 emits standard light for characteristic measurement set to the quantity of light of 1sun (=1000 W/m2) and spectrum of AM1.5, which conforms to Japan Industrial Standards (JIS). Illuminating and shielding of light on/from the solar cell 201 are performed by controlling open/close of the shutter 209 by the computer 207.
A method of measuring the voltage/current characteristic of the solar cell 201 using the above apparatus is explained below.
First, the light source 208 is warmed up and adjustment for emitting a standard quantity of light is performed. In this stage, the light source 208 is on and the shutter 209 is closed. Next, the solar cell 201, which is the object of the measurement, is set. Then, the shutter 209 is opened and the entire surface of the solar cell 201 is illuminated with the standard light. Under this condition, the computer 207 controls the DC current source 202 to output a voltage. The voltage to be applied across the solar cell 201 depends upon the type of the solar cell 201, and the optimum voltage is predetermined for each type of a solar cell.
In an operational sequence for measuring the voltage/current characteristic, while gradually changing output voltage of the DC power source 202, voltage values across the solar cell 201 measured by the voltmeter 205 and current values measured by the ammeter 204 are stored in memory of the computer 207.
After changing the output voltage of the DC power source 202 across the voltage range necessary for the measurement, the shutter 209 is closed, and the solar cell 201 is removed. Then, the voltage value data and the current value data stored in the memory of the computer 207 is graphed using a proper software, and a voltage/current characteristic curve as shown in FIG. 3 is obtained.
FIG. 3 is a graph showing an example of voltage/current characteristic of the solar cell 201. In FIG. 3, V in the abscissa shows voltage, and I in the ordinate shows current. A curve C in FIG. 3 is obtained by connecting points representing the measured voltage values and current values plotted on the graph. A point K on the curve C is the point where the product of the voltage and the current becomes maximum, i.e., the point where maximum electric power is taken out, and generally called the optimum working point. The electric power taken out at the optimum working point is the rated power.
Photovoltaic power generation has been rapidly spreading recently. In a residence, a solar panel is often installed on the roof, for instance, and in an isolated island, a solar panel is often installed on a rack. In order to reduce the number of steps for installing the solar panel, a photo-sensing surface of each solar cell tends to be broadened. For measuring characteristics of such a solar cell having a broad surface, a light source capable of illuminating an area corresponding to the entire surface of the solar cell is necessary; however, it is very hard to obtain a light source of that kind.
Generally, a Xenon lamp is most widely used as the light source, and a solar simulator using the Xenon lamp is used. However, the price of the solar simulator increases rapidly as the area that the Xenon lamp can illuminate increases. The increase in price is caused since it becomes harder to manufacture an air-mass filter and a condenser lens, both included in a solar simulator, as their sizes become larger, and the required output power from a power source for the lamp is extremely large. As a practical fixed-light type solar simulator, one having a light source capable of illuminating an area of about 50 cm by 50 cm is the largest on the current market. There is a simulator, capable of illuminating an area of about 1 m by 1 m, in which a lamp, the light source, pulses to decrease the required output power of a power source for the lamp. However, since the sizes of parts other than the power source for the lamp are the same as those of the fixed-light type solar simulator, such a solar simulator is also very expensive.
Thus, the solar simulator for measurement and test having a light source is expensive as described above; therefore, manufacturing cost of a solar cell increases. Accordingly, a method capable of measuring the characteristic of a solar cell having a broad surface at low cost is desired earnestly. Further, regarding a solar cell having a photo-sensing surface much greater than 1 m by 1 m, since a light source capable of illuminating such a broad area is not available, it is not possible, practically, to measure the characteristic of the solar cell.
The present invention has been made in consideration of the above situation, and has as its object to provide a measuring apparatus and method of measuring a characteristic of a solar cell having a large photo-sensing surface at low cost.
According to the present invention, the foregoing object is obtained by providing a measuring method of measuring a characteristic of a solar cell comprising the steps of: measuring a first characteristic of the solar cell while illuminating a predetermined area of a photo-sensing surface of the solar cell, wherein an area of the photo-sensing surface which is not illuminated is called a dark area; measuring a second characteristic of the solar cell in a dark state in which the photo-sensing surface is shielded from light; calculating a third characteristic by multiplying the second characteristic by an area ratio of an area of the dark area to an area of the photo-sensing surface; and calculating a characteristic of the predetermined illuminated area on the basis of the first and third characteristics.
Further, according to the present invention, the foregoing object is also attained by providing a measuring method of measuring a characteristic of a solar cell comprising the steps of: segmenting a photo-sensing surface of the solar cell into a plurality of blocks each having a predetermined area; measuring a first characteristic of the solar cell while illuminating each of the plurality of blocks, wherein an area of the photo-sensing surface which is not illuminated is called dark area; measuring a second characteristic of the solar cell in a dark state in which the photo-sensing surface is shielded from light; calculating a third characteristic by multiplying the second characteristic by an area ratio of an area of the dark area to an area of the photo-sensing surface; and calculating a characteristic of each of the plurality of the segmented block on the basis of the first and third characteristics of the corresponding block.
Furthermore, according to the present invention, the foregoing object is also attained by providing a manufacturing method of manufacturing a solar cell comprising a measuring step of measuring a characteristic of the solar cell in accordance with either of the above measuring methods.
Further, according to the present invention, the foregoing object is also attained by providing a measuring apparatus for measuring a characteristic of a solar cell, comprising: a light source for illuminating a photo-sensing surface of the solar cell; and a mask, provided between the photo-sensing surface and the light source, for limiting an area to be illuminated on the photo-sensing surface to a predetermined area.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.